Cho $\triangle{ABC}$ cân tại $A$. Vẽ $AH \perp BC \; (H\in BC)$. Gọi $M, N$ lần lượt là trung điểm của $AC, AB$.
Vẽ $MK\perp BC \; (K \in BC)$, $NH$ cắt $BM$ tại $O$, trung trực của $MH$ cắt $MK$ tại $I$.
Chứng minh : $\widehat{AOI} = 90^\circ$.
*** Cannot compile formula:
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*** Error message:
Error: Nothing to show, formula is empty
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